The objective of this Program is to develop a fundamental physical and chemical understanding of the mechanisms by which RNA molecules and their complexes with proteins carry out their biological functions. During the next five years, several different systems that are involved in the processes of protein synthesis by ribosomes, catalysis by RNA enzymes, and ribo-switches will be studied. While a major technique to be used will be single crystal X-ray diffraction, these structural studies will be integrated with genetic, biochemical, chemical and computational approaches. A major goal will be to capture the structures of these macromolecular machines at each step of the various processes they carry out, enabling the production of movies showing the molecular motions involved in these mechanisms. Of special interest are the motions that occur in the course of protein synthesis as the ribosome proceeds through its elongation cycle, the mechanisms of riboswitches and other RNAs using allosteric mechanisms, the mechanisms of catalysis by various ribozymes. Also of interest will be the ways in which the structures and properties of RNA molecules can be utilized to carry out various biological functions that are often analogous to those performed by proteins.